The purpose of this study is to identify risk factors associated with infection with the Human T-Cell Leukemia Virus (HTLV) in an endemic population in order to elucidate the origin, reservoir, and mode of spread of the virus. HTLV is the first oncogenic retrovirus which has been isolated from humans, and is closely associated with adult T-Cell Leukemia/Lymphoma in endemic populations. In addition, antibodies against the HTLV have been found to be much more prevalent among homosexual AIDS patients than among unaffected homosexual controls. The method to be used is that of a case-control study. This will be conducted in conjunction with population screening for antibodies by Dr.\Myron Essex in Kyushu, Japan. The population of Kyushu is particularly appropriate for this study because of the relatively high prevalence of antibodies against HTLV in the adult population (16%), as well as the well established collaboration between Dr. Essex and Dr. Nobuyoshi Tachibana of Miyazaki Medical School. For the study, "cases" will include approximately 200 individuals within the screened population who have antibodies against the HTLV. Controls will include 400 individuals without antibodies matched for age, sex, and residence. In addition, close relatives of cases will be included when available. All subjects will be interviewed for a variety of potential risk factors. These will include: contact with animals or animal products, marital status and family size, sexual behavior, occupation, religious practices, and food habits. Gaining understanding of the origin, reservoir, and mode of spread of HTLV is of significance in determining whether any preventive measures are feasible in the control of HTLV-associated malignancy. This study may also contribute to the understanding of the etiology of AIDS.